The present invention relates to an ozone generator for industrial use, such as water supply and sewage treatment, pulp-bleaching, and the like, which require a large amount of ozone.
A well-known ozone generator for industrial use changes an oxygen-containing feed gas to an ozonized gas or ozone gas by using silent discharge and is disclosed in Japanese Laid-Open Patent Publication No. 2-184506 (U.S. Pat. No. 5,034,198) or Japanese Laid-Open Patent Publication No. 9-315803, filed by the applicant of the present invention.
FIGS. 8(a) and 8(b) illustrate the structure of a conventional ozone generator disclosed in Japanese Laid-Open Patent Publication No. 9-315803. In FIG. 8(a), reference number 1 designates a gas chamber formed of stainless steel or similar material having high corrosion resistance to ozone, and reference number 2 designates ozone-generating pipes arranged in a central portion of the gas chamber 1 to be parallel to each other.
The gas chamber 1 is formed of a cylindrical or rectangular hollow body portion 1a, end plates 1b occluding opposite ends of the body portion 1a, and a pair of support plates 1c laterally spaced at an interval in order to support all parallel ozone-generating pipes 2 in the central portion of the body to maintain their horizontal position. The body has a feed gas chamber 1d formed between a left-end portion or plate 1b and the support plate 1c, and an ozonized gas chamber 1e between the right-end portion and the support plate 1c. A water jacket if is formed between the pair of the support plates 1c to cool the ozone-generating pipes 2. Furthermore, the body portion la has on its peripheral surface a feed gas inlet 1g, an ozonized gas outlet 1h, and an inlet 1i and an outlet 1j for ozone-cooling water, which are lead to the water jacket 1f. The body portion 1a and the end plate 1b are coupled via gaskets or air-tight seals, such as O-rings, using screws.
As shown in FIG. 8(b), an ozone-generating pipe 2 is formed of a tube shaped ground electrode 2a made of ozone-resistant stainless steel, a glass dielectric layer 2b lying on an inner surface of the ground electrode 2a, and a high-voltage electrode 2d of a hollow structure placed concentrically inside the ground electrode 2a to oppose to the dielectric layer 2b with a discharge gap 2c located between the high-voltage electrode 2d and the dielectric layer 2b. The ozone-generating pipes 2 are held between the support plates 1c of the gas chamber 1 and penetrate the support plates 1c laterally. Both ends of the ozone-generating pipe 2 respectively open to corresponding openings for the starting gas chamber 1d and the ozonized gas chamber 1e, which are formed in the gas chamber 1.
In addition, feeding leads 3, drawn out from each end surface of the high-voltage electrodes 2d of the ozone-generating pipes 2 inside the body of the gas chamber 1, i.e. feed gas chamber 1d, are connected to an external high-frequency power supply 5 via a bushing 4, which is provided on a peripheral surface of body portion 1a.
As a water-coolant system for the ozone-generating pipe 2, cooling water is supplied through an external cooling water circulating line 6 to the water jacket if of the gas chamber 1 and to each hollow-structured high-voltage electrode 2d of the ozone-generating pipes 2. 6a is a heat exchanger interposed between the water-cooling system and a secondary water-cooling system, 6b is a circulating pump, 6c is an ion exchanger, and 6d are manifolds. Cooling water conduit (insulating tube) 6e connected to the high-voltage electrodes 2d are drawn out to the ozonized gas chamber 1e in the body, and are branched and connected to the manifolds 6d.
As shown in FIG. 9, the ozone generator is combined with an oxygen production apparatus (feed gas source) 7 and an ozone-processing apparatus 8 (water supply and sewage treatment facility, or pulp-bleaching treatment facility) to introduce and supply a feed gas through a gas line 10 located between the feed gas inlet 1g of the gas chamber 1 and the oxygen production apparatus 7, and to supply the ozonized gas through a gas line 11 located between the ozonized gas outlet 1h and the ozone-processing apparatus 8. Reference number 12 designates a compression apparatus (pump) for compressing and transferring the ozonized gas.
With this structure, the oxygen-containing feed gas is introduced from the oxygen production apparatus (feed gas source) 7 into the body of the gas chamber 1 through the gas line 10, and is then discharged into the feed gas chamber 1d shown in FIG. 8(a). The gas is then distributed to the ozone-generating pipes 2, where the gas flows into the discharge gaps 2c. As the gas flows, a high-frequency voltage is applied to an area between the high-voltage electrode 2d and the ground electrode 2a, causing a silent discharge between the two electrodes and ozonizing a part of the feed gas, which then flows into the ozonized gas chamber 1e. In addition, the ozonized gas flown into the ozonized gas chamber 1e is supplied to the ozone-processing apparatus 8 through the gas line 11, which is connected to the ozonized gas outlet 1h.
An exhaust valve (not shown) is connected to the gas line connected to the gas outlet 1h, and the exhaust valve is adjusted to control the operating pressure of the ozone generator--that is, to adjust the pressure of the feed gas to a certain level, e.g. 0.17 MPa, for example. For example, when the ozonized gas generated by the ozone generator is to be used for a water supply treatment, the operating pressure of the apparatus is maintained at 0.17 Mpa. For pulp-bleaching treatment, the compression apparatus 12 (often a water-sealing pump) is installed in the middle of the gas line 11, as shown in FIG. 9, to increase the pressure of the ozonized gas to approximately 1 MPa before supplying it to the end use. Recently, the capacity and concentration of the ozone generators have increased as the scale of ozone-processing facilities has grown, due to the application of ozone for water supply sterilization and deodorization. However, it appears that as the ozone concentration generated in the ozone generator becomes higher, there raises the problems, which are described below.
Namely, when a voltage applied to the ozone-generating pipe is increased to produce thick or highly concentrated ozone efficiently, variation in the power supply voltage or the nature of the feed gas causes electric discharge in the discharge gap to shift from silent discharge to spark discharge, which ignites and decomposes ozone to thereby increase the gas pressures in the discharge space, thereby propagating flame outward. This phenomenon may be induced by ignition resulting from the system-generated static electricity, or by the ignition of an externally heated surface.
A graph in FIG. 10 shows the results of experiments on ozonolysis or ozone decomposition performed by the inventors. During the course of these experiments, ozonized oxygen of high ozone concentration was sealed in a closed container at an absolute pressure between 0.1 and 1.5 Mpa. An electric current was conducted through a nichrome wire placed inside the container to heat. With the heat of the nichrome wire acting as an ignition energy, ozone was subjected to decompose by itself, i.e. autolysis, producing rapid increase of a gas pressure in the closed container. If a gas pressure ratio is defined as the ratio of a maximum pressure to an initial pressure, the gas pressure ratio begins to increase rapidly when an ozone concentration approaches 220 g/Nm.sup.3 (in terms of 0.degree. C. and 1 atom.), and reaches a value between 3.0 and 3.5 at approximately 250 g/Nm.sup.3 or more. This trend is applicable to cases in which the sealing pressure of ozonized oxygen is changed from 0.1 to 1.5 MPa.
The ozone decomposing or ozonolysis mechanism is described in the thesis "THE OZONE TO OXYGEN FLAME (A. G. Streng and A. V. Grosse): 6th Symp (Int) on combustion No. 32 (1957). That is, as represented by formulae (1) to (3), ignition energy causes ozone to be decomposed into oxygen molecules and atoms (formula (1)), and ozone then joins with oxygen atoms to generate oxygen molecules (formula (2)). Finally, 1-mol ozone is decomposed into 1.5-mol oxygen molecules, and a 63.5-kcal exothermic reaction occurs (formula (3)). The resulting heat increases the gas molecule temperature and induces decomposition flame to thereby increase a gas pressure within the container. In addition, once this reaction produces decomposition flame, a decomposition chain reaction propagates flame from the ignition point to peripheral areas. EQU O.sub.3.fwdarw.O.sub.2 +O+34 kcal (1) EQU O+O.sub.3.fwdarw.2O.sub.2 +93 kcal (2) EQU O.sub.3.fwdarw.1.5O.sub.2 +63.5 kcal (3)
When ozone autolysis occurs inside the ozone generator and gas pressures increase as flame propagates, the following problems may occur to hinder use of the ozone generator; the ozonized oxygen, which is toxic to human, may leak between the body portion 1a and the end plate 1b of the gas chamber 1 of the ozone generator illustrated in FIG. 8(a); the gas pressure may cause the ozone-generating pipes 2 to shift from the predetermined positions, so that the feeding leads 3 of the high-voltage electrodes contact the body portion 1a of the gas chamber 1, resulting in a short circuit; the gas in the system may be forced back to the feed gas source; and the ozone generator may be subjected to the gas pressure higher than that it was specified. If the internal structure of the ozone generator includes a flammable organic material, decomposition flame may lead to a fire.
The present invention has been made in view of the above problems. A primary object of the invention is to obviate the above problems and to provide an improved ozone generator capable of preventing outward propagation of decomposition flame of ozonized gas induced by spark discharge in an ozone-generating pipe to thereby establish safety of the ozone generator.